1. Field of the Invention
The invention concerns the chaining arrangements between intermediate modules, particularly of repeater type, for a communications installation including transceiver terminals communicating by means of a common packet transmission link to which they are linked via the modules. It also concerns the communications installations equipped with such arrangements.
2. Description of the Prior Art
There are many communications installations including transceiver terminals which communicate with each other by means of a common packet transmission link which they utilize on a time-shared basis, each transmitting according to its requirements, when the link is available. These installations typically include intermediate modules responsible for connection operations for one or more transceiver terminals to which they are assigned.
For various reasons, and particularly for reasons of cost, these modules often have a concentration function and are therefore shared by several transceiver terminals. In addition, these modules often further ensure a standardization of the signals transmitted between a transmitting terminal and a receiving terminal by regenerating these signals so that they are always comprised within given admissible limits.
There are modules referred to as repeaters which are achieved so as to ensure such functions in the case of common packet transmission links such as those considered above, particularly in the case of the links provided to achieve local networks, such a standard type network, for example.
To fully profit from the transmission possibilities offered by a common link shared in this manner within the scope of a single installation, it is known to control the unit which is formed by the modules and the common link to which they are connected via an auxiliary network of control links linked to the modules. In a known embodiment, each module includes an input provided to receive an access monitoring blocking signal, of binary type, indicating whether the common link is accessible to it or not and whether it can thus utilize it to transmit data coming from a transceiver terminal to which it is associated. Each module also includes an output to transmit the access monitoring signal, when it itself wishes to access the common link so as to transmit from it.
It is therefore known to handle the access problems of modules of this type connected to a common packets transmission link through the serial chaining of the modules which is obtained by linking the access monitoring signal output of one module to the access monitoring signal input of another located downstream, and so forth, starting from a first one amongst them. The latter practically has priority when it wishes to transmit, when a transmission is not already in progress, it being understood that any module is fit for determining whether there is a transmission in progress on the common transmission link, any transmission being on principle transmitted by the common link towards all of the modules linked to it.
This solution makes it possible to achieve the auxiliary network of control links in a simple manner, since a single control link is sufficient to link the access monitoring signal input of the module with the access monitoring signal output of the module located upstream of it in the chain, if such a module exists, and since the module located upstream in the chain can constantly receive the access monitoring signal, of binary type, informing it that the link is accessible to it, in the absence of a transmission in progress.
There is nevertheless a so-called risk of collision if several modules simultaneously attempt to transmit after having determined on a separate basis that there was no transmission in progress on the common link. There is therefore provided a procedure, conventionally referred to as a restraining procedure, making it possible to make sure that only one of the modules having simultaneously attempted to go into transmission on the common link will be the source of the collision signal transmitted to all of the modules. This procedure and the means making it possible to implement it at the module level will not be treated herein as it is only indirectly related to the object of the present invention.
The chained auxiliary network solution mentioned above is only suitable when the number of chained modules is limited, since it introduces transmission times for the access monitoring signals which quickly become unacceptable. As an example, we will recall that the IEEE 802.3 standard provides that the delay in the transmission of a packet must be inferior to eight bit times, i.e. 800 nanoseconds, in the most unfavourable case, between two ports, one being the input port and the other the output port, for terminals linked together through a packet transmission link, via modules of repeater type, such as considered above. In addition, the operation of the unit serviced by the chained modules can be seriously disturbed in the case that the chain is interrupted, in the event of the failure or break of a link, for example, as well as in the event of the malfunction or the possible removal of a module.
It is also known to achieve an auxiliary network of control links avoiding the disadvantages mentioned above and making it possible to bypass the restriction in the number of modules caused by the chaining mentioned above. In this case, the access of the modules to a packet transmission link which they have in common is controlled by an external arbitration logic to which the respective inputs and outputs of the access monitoring signals of the different modules are individually linked. There is then no longer a chaining, which eliminates the delays which it caused, but the auxiliary network of control links involves the use of two wires for the access monitoring signals between the arbitrator logic and each of the modules.
But such a solution is not necessarily suitable for all installations since the wiring which it requires may not correspond to the wiring scheme planned for the installation as a whole. This is particularly the case when it is systematically planned to connect all the units of an installation by means of an interconnection network made up of point-to-point type links which are each physically structured and time-controlled according to the requirements of the two units which each of them links. Such a network is then capable of being subdivided into subnetworks respectively made up of point-to-point type links identically structured and controlled, each grouping units communicating in an identical manner. It includes, for example, at least one subnetwork of links utilized in packet mode, as in the previous case. In a known embodiment, a backplane board carrying appropriate point-to-point links then makes it possible to interconnect boards corresponding to units communicating with each other according to one same mode and in particular modules, such as mentioned above, for the control of these modules.